Hydraulic fracturing is a technique used to increase the permeability of an oil or gas-bearing zone in a well by forming a large number of cracks, or fractures, in the zone through which oil or gas can travel. Each such identifiable "zone" generally includes sediments that may be of sedimentary origin. In hydraulically fracturing the zone, a fluid, typically compressible, is injected into a well and the pressure from the weight and/or compression of the fluid causes the zone to crack or fracture. A single well typically accesses multiple zones located at different depths. Each zone is injected with a granulated material (e.g., proppant) to fill and restrict closure of the fractures therein.
Most of the methods for hydraulically fracturing isolate other zones from the slurry so that the slurry contacts only the zone to be fractured. In one method, a production casing is cemented to the wall of the wellbore for wellbore stability and/or zone isolation from other fluid-producing sediments. The cemented casing is perforated at or near the formation face by using a wireline or similar device to position and ignite an explosive device, such as a perforating gun. Proppant laden slurries are injected into the casing at high pressure to flow through the casing perforations and hydraulically fracture the zone. In another method, part of the production casing is cemented to the wellbore with the part of the casing adjacent to the zones being uncemented. The individual zones are isolated by straddle packers, the casing perforated as described above and each zone hydraulically fractured.
The above-noted methods generally can fracture only one zone at a time which entails a repetition of a series of steps zone by zone. Such a multiplicity of steps not only is costly but also prolongs the time to drill and complete the well.
In the above-noted methods, the contacting of the slurry with one or more previously fractured zones, especially shallower zones, can impair the ability of the slurry to fracture another zone. To seal off fractured zones from the proppant-containing slurry, deeper zones can be fractured first and a bridge plug, straddle packer, or a layer of sand used to cover and seal the zone. The steps of positioning the bridge plug or straddle packers or adding and removing the sand, however, introduce additional steps into the already lengthy well completion process. It is also difficult to correctly position the bridge plug, straddle packers, or sand layer.
In the perforating step, additional complications can arise as it is often difficult to properly position the device adjacent to the zone to be fractured. If the perforations are improperly positioned, little, if any, of the zone would be fractured. For example, in wide zones (e.g., 5,000 to 10,000 feet) improper positioning of the perforations can cause the fractures to be concentrated in isolated portions of the zones that are adjacent to the casing perforations or have a weaker resistance to fracturing than other portions of the zone. The concentration of fractures in specific portions of the zones can reduce the ability of oil and/or gas in unfractured portions of the zone to flow into the well.
The time to complete a well by the numerous steps of the above-noted methods is considerable (e.g., several days) and can cause liquids in the slurry to "waterlog" the formation. The extended period to complete the well can permit the water in the fracturing slurry to enter into the zones. The presence of the water in the zone can decrease production of oil and/or gas from the zones.
After formation of the fractures by hydraulic fracturing techniques such as the above-noted methods, there are often problems with the filling of the fractures with a sufficient amount of a proppant (e.g., sand) to restrict closure of the fractures. If too little proppant is placed in the fractures, the fractures will close and the permeability of the zone will not be improved significantly. If too much proppant is injected into the well to ensure that the fractures are filled with proppant, an excess amount of proppant must generally be removed from the well which can entail great expense.